In recent years, green vehicles that can substitute for existing internal-combustion engine vehicles are actively developed due to a high oil price, regulations of carbon dioxide emissions, and the like. An electric vehicle traveling by driving an electric motor and a hybrid vehicle using an internal-combustion engine and an electric motor as a driving source have been or will be commercialized according to manufacturers.
An in-wheel motor vehicle, which has an electric motor (driving motor) mounted to each wheel using power of batteries as a power source, is being developed among electric vehicles. The in-wheel motor vehicle is a vehicle in which power is transmitted directly to each wheel by the electric motor disposed inside a wheel rim of the wheel.
Thus, in a case where the in-wheel motor is applied to an electric vehicle, a power transmission apparatus such as a transmission or differential gear can be omitted, so that it is possible to decrease the weight of the vehicle and to reduce energy loss caused by a power transmission process.
Recently, an electric bus having an in-wheel motor technique applied thereto (referred to as an ‘in-wheel electric bus’ hereinafter) has been developed. The in-wheel electric bus uses a high voltage (e.g., 486 to 784V) as compared with an ordinary electric vehicle.
Accordingly, a relay in a battery system is not turned off due to its fusion frequently in a hybrid electric vehicle (HEV), a plug-in hybrid vehicle (PHEV), an electric vehicle (EV), etc., using a high voltage, which is undesirable.
Accordingly, the high voltage in the battery system is not released, or a fire or the like occurs due to heat generated in a resistance load in the fusion of the relay.
Thus, when a high-voltage battery is re-connected to a load such as each in-wheel motor (i.e., an inverter in a motor controller) in a vehicle driving system (e.g., in a restart after idle stop), whether the relay is turned off is checked by verifying a remaining current and its charging state in the system, particularly before each relay of a power relay assembly (PRA) is turned on. Here, the PRA performs switching so that the power of the battery is selectively supplied to an electric load in a vehicle.
Typically, whether a relay is fused or not is verified by checking a DC-link voltage in an inverter for in-wheel motor as shown in FIG. 1 before a high-voltage battery (main battery) is connected to an inverter of each in-wheel motor (driving motor) (S2). In a case where it is verified that the DC-link voltage in the inverter is less than a reference voltage in the checking process, a pre-charge relay and a main relay (high-voltage (−)/(+) relay) are sequentially turned on (S3 and S5).
However, it takes a long time to check whether the relay is turned off by verifying the discharging state of remaining current until the remaining current in the system is discharged, i.e., until the DC-link voltage in the inverter is less than the reference value.
Accordingly, it takes a long time to check the status of the relay whenever the battery is connected to the in-wheel motor, since the in-wheel electric bus starts after the idle stop. This lengthens the start time of the in-wheel electric bus, which is undesirable.
FIG. 2 is a graph showing a state in which remaining current drops in a conventional system. Here, the vertical axis represents a remaining voltage (V) in the system, and the horizontal axis represents time (sec).
As shown in FIG. 2, it takes a long time until the remaining voltage gradually drops and then becomes less than the reference value. Hence, there is a problem in that inspection waiting time and control time are lengthened when the in-wheel electric bus starts.
A method of setting the reference value (V), at which the relay is re-joined, to be high is used for the purpose of shortening a start time of the in-wheel electric bus. In this case, although the relay is substantially fused, an instantaneous discharge is generated by another variable. Hence, there may exist a state in which the DC-link voltage in the inverter is less than the reference value (set to high as described above). Therefore, a need exists for performing a process of additionally verifying the state. Further, the number of variables increases, and therefore, the control becomes complicated.
Alternatively, a method of adding a discharging resistor to a single component in the system is introduced. In the method, a discharging resistor and a cable are additionally connected to a single component in the driving system of the in-wheel electric bus, e.g., a motor control unit (MCU), auxiliary machinery inverter, low-voltage DC/DC converter (LDC), high-voltage junction box, etc. The method, however, increases the number, size or price of the single component Therefore, the method has a negative influence on the optimization of the system.